Thermochemical treatment, in halogenated atmosphere, of a carbon-containing material

ABSTRACT

The invention concerns (1) thermochemically treating by pack-cementation a carbon-containing material, which may have an open porosity, to generate a refractory carbide coating on its surface and, if the material is porous, within the material; and (2) the use of specific alloys as a pack for thermochemically treating carbon-containing materials, optionally with an open porosity, in a halogenated atmosphere. Pack-cementation is carried out under reduced pressure using an element E (to be transported and to be reacted with the carbon in the material to generate the expected carbide) alloyed to an element M, and using a halide (chloride or fluoride, preferably a fluoride) of the same element M, of low volatility, present in the solid form.

The present invention relates to:

thermochemically treating a carbon-containing material which canoptionally have an open porosity, to generate a refractory carbidecoating on the surface and within said material if it is porous, bypack-cementation (carbiding);

the use of specific alloys as a pack (cement) to thermochemically treatcarbon-containing materials, which may optionally have an open porosity,in a halogenated atmosphere.

In a first aspect, the present invention proposes an efficient method ofgenerating refractory carbide coatings on the external, and on theinternal surfaces when they exist and are accessible, ofcarbon-containing materials. Forming this type of coating is of greatimportance in numerous fields since such coatings endow suchcarbon-containing materials with high resistance to wear, ablation,erosion, oxidation, and corrosion. Such coatings can also protect suchcarbon-containing materials from the diffusion of elements within them.Said coatings can also improve the moistening of carbon-containingmaterials by molten metals. In particular, the method of the inventionhas been developed for producing particularly effective thermal shieldsand barriers to diffusion.

The method of the invention is a pack-cementation (carbiding) method: itgenerates a refractory carbide on the surface (surface=externalsurface+, possibly, the internal surface) of a carbon-containingmaterial from the carbon C of said heat treated material and from anelement E supplied in the form of a pack (cement) in the reaction mediumand transported in the form of a halide to the surface of saidcarbon-containing material. Said method of the invention jointly uses anE—M type donor pack alloy (more precisely xE-yM-zM′) and a solidactivating compound with formula MX_(n); under conditions, notably ofpressure, in which said element E can be transported (where compoundMX_(n) is sufficiently stable for halides MX_(n), in the solid and gasforms, to coexist with EX_(n), in the gas form). Said conditions and thenature of elements E, M, M′, X are defined below.

A pack-cementation method has already been described in French patentapplication FR-A-2 304 590. That method consists of treating thecarbon-containing material to generate a coating of a carbide of arefractory metal at its surface:

at a temperature in the range 850° C. to 1250° C.;

at atmospheric pressure, in a hydrogen-containing atmosphere;

in the presence of a pack powder comprising an intimate mixture of therefractory metal (Ti, Zr, Hf, Ta, Nb) and a halide of that refractorymetal (TiCl₄, ZrCl₄, etc.) or a halide which, in situ, can generate thehalide of that refractory metal (ammonium halides (volatile), or cobalt,nickel, iron, or aluminium halides).

That method is carried out using the material to be coated in contactwith (in) the pack powder which comprises a refractory diluent (alumina,magnesia) and chromium (catalyst), in addition to the refractory metaland the halide of that refractory metal, or a precursor thereof.

In the prior art method, the refractory metal (E) is transported by itsown halide (EX_(n)), introduced directly or generated in situ from aprecursor of that halide (M′X_(n)) of which the halogen is displaced(precursor M′X_(n) does not remain solid, it is only used to generateEX_(n)).

The method of the invention can be analyzed as an improvement over, oran optimization of, that prior art method. The method of the inventionproduces very good results with a very large range of carbon-containingmaterials which are non porous, or slightly porous, or even very porous(the prior art method does not enable suitable coatings to be producedin the internal portions of the treated parts, in the pores of suchparts . . . the thickness of the deposit is observed to be non uniform,in particular because diffusion in the gas phase is too slow) and with alarger range of elements E including metalloids such as boron andsilicon, in addition to metals such as titanium, zirconium, hafnium,tantalum, niobium and chromium. The very good results obtained withboron in particular should be mentioned. It should also be notedincidentally that the commercially available packs recommended forboriding steels do not enable boron to be transported to the surface ofcarbon-containing materials.

The prior art also describes the production of coatings in the internaland external portions of metal parts, by suppling a metal such asaluminium (FR-A-2 576 916 and FR-A-2 576 917). The method described inFR-A-2 576 916 employs a gas phase flowing between the inlet and outletof a reactor. To transport the supplying metal (E), the method describedin FR-A-2 576 917 uses a solid halide of that supplying metal(EX_(n)=AlF₃, CrF₂ or CrCl₂) or a solid alkali halide (NaX, KX, forexample) (in contrast to the method of the present invention which usesa halide of a metal M alloyed to E, of the type MX_(n)). In any case,the methods of FR-A-2 576 916 and FR-A-2 576 917 were developed in acompletely different context to that of the present invention. Thosemethods generate coatings which are not carbides. Generation of thosecoatings does not use carbon migration within the treated material.

The present invention proposes an efficient method of thermochemicallytreating non porous, slightly porous or even highly porouscarbon-containing materials (in other words, carbon-containing materialswhich may optionally have open porosity) in a halogenated atmosphere(pack-cementation) to generate refractory carbide coatings at thesurface of said materials (surface=external surface+optionally internalsurface). Said method can produce coatings with a morphology that isregular (particularly in terms of thickness and nature of the phases)which in the case of porous materials can be measured in terms ofuniformity (the ratio, expressed as a %, between the thickness of thecoating over the central zone of the treated material and the thicknessof the coating on the external surface of said treated material). Saidmethod, controlled, can generate coatings with uniformity of more than70% in some types of carbon-containing materials with a large openporosity. Such results could not be obtained using the methods of theprior art, where the uniformity of the coatings hardly ever exceed 10%.

The method of the invention comprises maintaining the carbon-containingmaterial:

a) at a temperature in the range 700° C. to 1300° C.;

b) at a reduced pressure, in the range 0.1 kPa to 30 kPa, of hydrogen, arare gas or a mixture of such gases;

c) in the presence of a donor pack constituted by at least one element Eselected from titanium, zirconium, hafnium, tantalum, niobium, chromium,silicon, and boron, alloyed to an element M selected from aluminium,calcium, chromium, yttrium, and magnesium, and optionally alloyed to amoderator element M′; said moderator element M′ being necessarily usedif E=M=Cr and then being other than chromium;

and a solid activating compound of low volatility at said treatmenttemperature, with formula MX_(n), where X consists of chlorine orfluorine, advantageously fluorine (and n, a whole number, corresponds tothe valency of element M).

According to the invention, pack-cementation is carried out at reducedpressure using element E (to be transported and to react with the carbonof the material to generate the expected carbide) alloyed to an elementM, on the one hand and a halide (chloride or fluoride, preferablyfluoride) of the same element M, of low volatility, present in the solidform, on the other hand.

The pack used contains at least one element E alloyed with an element M.Said pack generally contains a single element E but a plurality ofelements is not excluded. It is recommended that boron and silicon areused jointly, to generate a coating constituted by the carbides of thesetwo elements, endowing the treated material with improved resistance tooxidation over a wider temperature range than that provided by a boroncarbide. Said pack is generally a binary E—M type alloy. It can consistof an E—M—M′ type alloy if a moderator element M′ is used to fix theactivity of said elements E and M in said pack. Said pack isadvantageously produced prior to carrying out the thermochemicaltreatment, at atmospheric pressure, in conventional manner. This is notcompulsory, however.

In any case, to generate a carbide of an element E on the surface of acarbon-containing material accordance with the invention, a suitableelement M must first be selected and, if necessary, so must an elementM′. Said element M is selected if a suitable halide MX_(n) exists(solid, of low volatility at the treatment temperature). The nature ofelements E and M being fixed (E≠M), binary E—M alloys can then beconsidered. If any exist within which the respective activities ofelements E and M enable transport and liberation of said element E onthe surface (external and possibly internal) of the treated material,then a priori a moderator element M′ is not required. If not, amoderator element M′ is recommended and thus an E—M—M′ type pack.

The moderator element M′ is in general a metal, advantageously selectedfrom iron, nickel, chromium, cobalt, molybdenum, and tungsten.

Clearly, if E=Cr, then M≠Cr, or if E=M=Cr, then M′≠Cr.

Surprisingly, using the elements listed above and summarized below:

E=Ti, Zr, Hf, Ta, Nb, Cr, Si, B and alloys thereof;

M=Al, Ca, Cr, Y, Mg;

M′=Fe, Ni, Cr, Co, Mo, W;

(it being understood that if E≠M, M′ may or may not be used and ifE=M=Cr, then M′ is used and M′≠Cr);

X=Cl or F (MX_(n) remaining solid at the treatment temperature);

it is possible, within the context of the invention, to prepare suitablepacks and activating compounds to effectively treat carbon-containingmaterials by carrying out a pack-cementation method (carbiding method)under the following conditions:

at a temperature in the range 700° C. to 1300° C.;

at a reduced pressure, in the range 0.1 kPa to 30 kPa, of hydrogen, arare gas or a mixture of these gases.

The treatment temperature, in the range 700° C. to 1300° C., isgenerally less than 1050° C. to obtain coatings of carbides of elementsE other than boron, but more than 1100° C., advantageously more than1200° C., to obtain boron carbide coatings.

It should be remembered that, in accordance with the invention, at saidtreatment temperature the halogenated activating compound MX_(n) remainssolid, in a condensed form (throughout the treatment period). It has alow vapor pressure at the treatment temperature; in any case, the vaporpressure is lower than the pressure at which the method is carried out.

Given that all of the chlorides of a given element are both less stableand more volatile than the fluorides, and that said chlorides also havelower melting points than those of the corresponding fluorides, itshould be understood that the halogenated activating compound used inthe method of the invention is advantageously a fluoride (MF_(n)).

Said halogenated activating compound MX_(n) is used in a quantity whichis sufficient for it to subsist in the solid state, when all of thehalides have been generated. Advantageously, an excess or even a largeexcess is used. Thus, by way of illustration, said halogenatedactivating compound can generally be used in an amount of about 5% byweight with respect to the weight of the pack.

The method of the invention is carried out at low pressure of aprotective gas.

The protective gas is used to prevent any oxidation, to confine thereaction medium, and to set the total pressure at that at which thetreatment is carried out. Said value is theoretically selected so as tobe much higher than the sum of the partial pressures of the halogenatedgaseous species, such that they are confined in the treatment chamber.As indicated above, said value is in the range 0.1 kPa to 30 kPa. It isadvantageously in the range 0.5 kPa to 15 kPa, more advantageously againin the range 0.5 kPa to 5 kPa. When operating below 0.1 kPa or above 30kPa, diffusion problems mean that it is difficult to produce suitablecoatings, in particular within porous carbon-containing materials.

The protective gas, which must enable element E to be liberated on thesurface of the treated materials, can consist of:

a rare gas such as helium or argon. These two light rare gases areparticularly preferred. In particular, it is recommended that helium beused, since this gas is the lightest, and can thus accelerate diffusionin the gas phase;

a reducing gas, such as hydrogen. The supplemental reduction reactionswhich can be made possible by the use of such a gas may be useful;

a mixture of these gases: a rare gas, such as helium or argon, and areducing gas such as hydrogen.

Regarding the duration of the thermochemical treatment of the invention,it can clearly be varied and is generally in the range from a few hoursto a few tens of hours. It clearly depends on the desired thickness ofthe carbide coating and on the difficulties of infiltration into theporous materials. The method of the invention can produce coatings witha thickness which can vary from a few manometers to a few tens ofmicrons.

The method of the invention, the essential features of which aredescribed above and for which advantageous variations are developedbelow, is suitable for treating any type of carbon-containing materials.The term “carbon-containing material” as used in the present descriptionand in the accompanying claims means materials comprising more than 25atomic % of carbon; said carbon can be free or combined, in particularin the form of a hemi-carbide such as SiC.

It should be remembered that a portion of the carbon in saidcarbon-containing material when treated in accordance with theinvention, is used to generate the carbide coating (in this regard, themethod of the invention is a cementation method which is very differentfrom a CVD or CVI type method) and that the method of the invention issuitable for treating non porous, slightly porous, or highly porouscarbon-containing materials. More precisely, the method of the inventionis suitable for treating both non porous materials and materials with anopen porosity in the range 2% to 98%, generally in the range 5% to 80%.It is particularly suitable for treating:

graphite parts, which are non porous or which have an open porositygenerally in the range 2% to 15%;

carbon/carbon composite materials, which have been completely orpartially densified, with an open porosity generally in the range 5% to15%; (on that type of materials which are relatively densified material,the invention principally aims to generate a superficial coating whichmay be quite thick).

fibrous preforms with a large open porosity, generally in the range 60%to 80%, and in particular non densified fibrous carbon preforms or nondensified SiC fiber based fibrous preforms (Nicalon® or the like),pre-treated to have a superficial carbon layer, or not pre-treated; (onthat type of non densified material, the invention generally aims toform a thin sheath over the fibers such that the mechanical propertiesof those fibers are modified only slightly);

carbon foams of very low density with an open porosity generally in therange 50% to 98%;

(on that last type of very porous carbon-containing material, theinvention can produce homogeneous deposits in the core of the foam. Thisthus produces a low density heat insulating material with goodmechanical properties at high temperatures).

Whatever the treated carbon-containing substrate, at any stage of themanufacture, the carbide layer generated increases the hardness,reinforces the resistance to oxidation and to chemical corrosion,constitutes a barrier to diffusion, in particular against oxygen, etc.

The method of the invention as defined above can be carried out in anumber of variations.

Regarding the relative disposition of the reactants—treatedcarbon-containing material, pack, activating compound—it should be notedthat:

The treated carbon-containing material can be at least partiallyimmersed in the pack or maintained in the gas phase in a zone close tosaid pack. Non porous or slightly porous materials are advantageouslytreated using said materials disposed in the pack (such a material/packcontact does not cause a problem of said pack adhering to said materialunder the implementation conditions of the method, and thus does notrequire a severe post-treatment to remove the treated material from saidpack) while treating porous or highly porous materials is advantageouslycarried out with said materials being maintained in the gas phase(without contact with the pack). In a further variation of the method ofthe invention, the pack is used in the form of a slip, which is appliedto the surface of the material to be treated. More precisely, thefollowing can be carried out:

firstly, a powdered pack is produced;

then a suspension of said powdered pack is prepared using a solvent (forexample water) and possibly a fugitive binder (for example polyvinylalcohol);

the prepared slip is applied (for example by painting) to the surface ofthe carbon-containing material to be treated (over all of its surface oronly to a portion thereof);

the material which is at least partially coated with said slip isthermochemically treated in accordance with the invention.

After cooling, the carbon-containing material has been transformed atits surface and at depth in its subjacent porosity (if it exists).

In particular, parts such as aircraft brake disks of carbon/carboncomposite can be treated to protect them against corrosion at leastlocally (on their friction surfaces in respect of the brake disks). Thecarbide layer generated can also be used as a bonding sub-layer for afurther protective layer;

The solid activating compound is advantageously not in direct contact,either with the pack or with the carbon-containing material. In aparticularly preferred variation, it is maintained at a distance fromthe pack/carbon-containing material ensemble at a temperature 20° C. to200° C. lower, more commonly 50° C. to 100° C. lower than thetemperature at which said carbon-containing material is treated(temperature of the pack/carbon-containing material combination). Thusthe risks of re-condensation of the activating compound MX_(n) on thesurface of the treated material are minimized. Carrying out the methodof the invention at a single given activating compound/pack contacttemperature is not completely excluded, in particular when treating nonporous or slightly porous materials with relatively volatile halides,but the inventors have learned that the best results, in particular withporous materials, are obtained when said activating compound, which isof low volatility, is maintained in the solid state, in a zone which isat a temperature θ_(A), while the pack/carbon-containing materialensemble (said carbon-containing material generally being at leastpartially in said pack or in the gas phase in a zone very close thereto)is maintained in a further zone at a temperature θ_(C): θ_(C)>θ_(A). Thetemperature difference, Δθ=θ_(C)−θ_(A) can, as is recommended, berelatively high. Thus such differences in temperatures, of more than100° C., are recommended for generating boron carbide coatings ofsubstantial thickness. This preferred variation for the method of theinvention with a thermal gradient, in which the solid activatingcompound is maintained in a zone which is “cold” with respect to the“hot” zone in which the carbon-containing material (and pack) istreated, is particularly original.

The method of the invention as described above is carried out using adonor pack: an E—M type or E—M—M′ type alloy. It is a true alloy, and isin no way a simple mixture of powders. The alloy may have been producedprior to carrying out the method of the invention, completelyindependently thereof. It can thus result from a known thermochemicaltreatment of powders (E, M, optionally M′) or from dividing ametallurgical alloy (obtained by a conventional metallurgical method)into particles with a large specific surface area. In a furthervariation, said alloy can be produced, when carrying out the method ofthe invention, in the apparatus provided for the method, prior to or asan integral part of said method, in the presence of an activatingcompound. In the first case, a mixture of suitable powders comprisingsaid activating compound is heat treated at atmospheric pressure. In thesecond, the method of the invention is commenced (at a reduced pressure)in the presence of a mixture of powder and the activating compound (saidmixture of powders being transformed into an alloy as the temperaturerises).

In general, the donor pack necessary to carry out the method of theinvention is advantageously produced prior to carrying out said method,at atmospheric pressure either by a thermochemical treatment carried outin the presence of a activating compound (treatment independent of orprior to the thermochemical treatment of the invention), or by ametallurgical type treatment followed by dividing the alloy obtained(treatment independent of the thermochemical treatment of theinvention).

In a particularly preferred variation, said pack is produced prior tothe treatment of the invention in an apparatus comprising a hot zone anda cold zone, the powders: E, M, optionally M′ and MX_(n), being mixed inthe hot zone and the halogenated activating compound MX_(n) then beingrecovered condensed in the cold zone.

At the end of the method of the invention—which end depends on the aim,i.e., the production of a given thickness of carbide layer(s)—withcertain donor elements E such as tantalum, niobium and chromium, it ispossible to obtain a plurality of carbides in a multi-phase coating.Such multi-phase coatings are important per se. It may nevertheless beopportune to subject the carbon-containing materials coated with saidmulti-phase coatings to a complementary heat treatment to transform theminto the corresponding mono-phase carbides; in principal, thesemono-phase carbides are more refractory.

Such annealing heat treatments, which can be qualified as diffusiontreatments, are familiar to the skilled person. They are carried out atatmospheric pressure in the absence of any halogenated activating agent.Thus it is possible to treat carbon-containing materials which have beencoated with a two-phase TaC+Ta₂C coating at a temperature of close to1300° C. after the thermochemical treatment of the invention totransform said two-phase coating into a mono-phase TaC coating.

Said TaC carbide is known to be the most refractory of carbides;coatings of this carbide are among the most effective as a diffusionbarrier.

The method of the invention as described above and illustrated below isadvantageously carried out under the following conditions to generatecoatings of zirconium carbide, tantalum carbide, or boron carbide.

Zirconium Carbide Coatings

Advantageously, the temperature is less than 1050° C., generally in therange 800° C. to 1000° C., at a reduced pressure of a rare gas selectedfrom helium and argon and advantageously consisting of helium, in thepresence of a Zr—Al donor pack and an AlF₃ activating compound. The raregas is preferred to hydrogen as it has been experimentally establishedthat with said rare gas, uniform coatings with a regular morphology areobtained. Irregular morphologies (a “scaly” surface) are obtained at thecore of preforms treated in hydrogen and can be damaged duringsubsequent densification thereof.

Tantalum Carbide Coating

The temperature is advantageously less than 1050° C., generally in therange 700° C. to 1000° C., at a reduced hydrogen pressure in thepresence of a Ta—Cr donor pack and a CrF₂ activating compound. In thiscase, the involvement of a reducing gas is highly beneficial. Asdescribed above, the carbide obtained after such a thermochemicaltreatment is two-phase (TaC+Ta₂C); it is optionally annealed to convertit into a mono-phase tantalum carbide (TaC) coating.

Boron Carbide Coating

The temperature is advantageously more than 1100° C., generally in therange 1200° C. to 1300° C., with a reduced hydrogen pressure, in thepresence of a B—Mg or B—Y donor pack and a MgF₂ or YF₃ activatingcompound (MgF₂ being combined with B—Mg and YF₃ with B—Y). In thiscontext, the use of a reducing gas is also highly beneficial,particularly when yttrium is used.

The good results obtained in the present invention with boron are, asalready pointed out, of special importance. They are relativelyunexpected, especially due to the fact that the electropositive natureof the element boron is a priori insufficient.

The variations for the method of the invention described above for theelements E=Zr, Ta or B are advantageously carried out using theactivating compound at a lower temperature than the temperature at whichthe carbon-containing material (and the pack) is treated, particularlywhen treating porous carbon-containing materials.

The present invention also provides an apparatus for carrying out thethermochemical treatment of the invention as described above. Theapparatus comprises:

a first, partially sealed, chamber in which the reactants are placed andin which they react; said first chamber is advantageously formed fromstainless steel or graphite;

a second chamber, in which said first chamber is located; said secondchamber is sealed against the ambient atmosphere and is associated withmeans for circulating within it hydrogen, a rare gas or a mixture ofthese gases at a reduced pressure;

heating means to maintain and control the treatment within saidchambers; said heating means advantageously being capable of maintaininga temperature difference of 20° C. to 200° C. between two zones in saidfirst chamber.

The ensemble comprising the material to be treated, the donor pack andthe activating compound is intended to be placed in said first chamber.It is recommended that the method be carried out in a first graphitechamber when the presence of iron in the reaction medium is to beavoided and/or when it is operated at high temperatures, for example toproduce boron carbide. When free of these constraints, it is recommendedthat the method be carried out in a first chamber of stainless steel.Said first chamber is kept partially sealed, generally using a plug,such that the reactive atmosphere is confined within it. Thus whencarrying out the method, a regime which is close to thermodynamicequilibrium can be established between the gas phase and the pack, andbetween said pack and the carbon-containing substrate; if the method islimited by solid phase diffusion, this is extremely desirable.

The first chamber can advantageously be provided with means formaintaining the material to be treated close to the pack without cominginto contact therewith and with boat type means to contain thehalogenated activating compound at a distance. With such means, physicalseparation of the treated carbon-containing material/pack and, ifdesired, physical separation of the treated carbon-containingmaterial/pack/halogenated activating compound is ensured. This latterphysical separation is advantageously combined, as described above, withsuitable heating means which can maintain a temperature differencebetween the zones where are localized on the one hand the treatedcarbon-containing material/pack (mixed or physically separated) and onthe other hand the halogenated activating agent.

The partially sealed first chamber is located in a second chamber whichis sealed from the exterior and in which a gas is circulated (hydrogen,a rare gas, such as argon or helium or a mixture of these gases) underreduced pressure.

Said first and second chambers are associated with heating means whichcan heat and maintain the treatment temperature required within. Asalready defined above, the heating means are advantageously capable ofmaintaining and controlling a temperature gradient between a cold zonewhere the halogenated activating compound is located and a hot zonewhere the pack and the carbon-containing material to be treated arelocated, either mixed together or physical separated.

In a variation, said first and second chambers are located in a furnacewith cylindrical geometry.

The invention also generally concerns the use of an alloy with formula:

xE—yM—zM′

where:

E is selected from Ti, Zr, Hf, Ta, Nb, Cr, Si, B and alloys thereof;

M is selected from Al, Ca, Cr, Y, Mg;

M′ is selected from Fe, Ni, Cr, Co, Mo, W;

x, y and z represent the atomic percentages of each of said elements E,M, M′;

 where

x≠0

y≠0

and z=0 or z≠0, given that:

if E=M=Cr, then z≠0 and M′≠Cr;

if E=Ti, Zr, Hf, Ta or Nb and M=Cr, then z≠0 and M′≠Cr,

as a pack for the thermochemical treatment, in a halogenated atmosphereof a carbon-containing material optionally with an open porosity.

The use of alloys of this type (in a divided form: powders, granules,pieces, etc.) as packs in cementation methods carried out in ahalogenated atmosphere, is novel. Said use under the general andadvantageous conditions described above can produce surprising resultsin particular with highly porous materials.

The various aspects of the invention are illustrated in the followingexamples.

EXAMPLE 1

Thermochemical treatment outside the pack, generating zirconium carbidelayers on three-dimensional (3D) fibrous carbon preforms with 75%porosity.

A mixture of aluminium and zirconium powders was placed in a semi-sealedstainless steel chamber in proportions of 38.2 atomic % (15.5% byweight) of Al and 61.8 atomic % (84.5% by weight) of Zr to which analuminium fluoride AlF₃ powder was added in an amount of 5% by weight ofthe charge. The ensemble was heated to 927° C. (1200 K) at atmosphericpressure maintained by a stream of hydrogen, for 36 hours. At the end ofthis treatment, an alloy (or pack), of aluminium-zirconium with acomposition of 38.2 atomic % of Al and 61.8 atomic % of Zr was obtainedwhich was in the form of a mixture of porous granules and powders; thealuminium fluoride had been completely displaced towards the coldestwall in the semi-sealed chamber where it had re-condensed.

A portion of said Al—Zr alloy was removed and introduced into the samesemi-sealed chamber into the portion which became the hottest portion. A3D fibrous carbon 20×15×5 mm³ preform with 75% porosity was suspendedabove the pack with no contact with the latter. The ensemble was placedin the middle portion of a jacketed tube furnace and heated to atemperature of 827° C. (1100 K) so that there was a difference of 100°C. between the hottest temperature where the part to be treated and theAl—Zr pack were located and the coldest temperature where the solid AlF₃was located. Under these conditions, the theoretical activities of thealuminium and zirconium in the alloy were a_(Al)=1.3×10⁻⁴ anda_(Zr)=3.3×10⁻¹. A 16 hour treatment at a total pressure of 2.67 kPamaintained by a stream of helium was carried out. At the end of thistreatment, an adherent zirconium carbide coating with a highly regularmorphology had been produced on all of the fibers constituting thepreform. Its thickness varied from 110 nanometers (nm) to 100 nm betweenthe exterior and the center of the part.

EXAMPLE 2

Thermochemical treatment outside the pack, generating zirconium carbidelayers on carbon/carbon composites with 10-15% porosity.

An Al—Zr alloy was produced in the presence of an activating compoundAlF₃ under the same conditions as those described for Example 1. In thisinstance, the composition of the powder mixture was 45.8 atomic % (20%by weight) of Al and 54.2 atomic % (80% by weight) of Zr.

A carbon/carbon composite part 20×15×5 mm³ with 10% to 15% porosity wassuspended above the prepared alloy (with no contact with the latter) andtreatment was carried out at 927° C. (1200 K) for a period of 100 hours.Under these conditions, the theoretical activities of the aluminium andzirconium in the alloy were a_(Al)=8.8×10⁻³ and a_(Zr)=2.9×10⁻². Thetotal pressure was 2.67 kPa, maintained by a stream of helium. Thetemperature difference between the treatment zone and the zone where theAlF₃ activating compound was solid was 50° C. The thickness of thezirconium carbide coating obtained, which was adherent and had a highlyregular morphology, varied from 2.9 μm to 1.8 μm between the exteriorand the center of the part.

EXAMPLE 3

Thermochemical treatment outside the pack, generating layers of tantalumcarbide on three-dimensional (3D) fibrous carbon preforms with 75%porosity—High temperature diffusion treatment generating mono-phasetantalum carbide coatings on three-dimensional fibrous carbon preformswith 75% porosity.

a) The following were simultaneously introduced into a semi-sealedstainless steel chamber:

into the portion which would have the hottest temperature, a mixture ofchromium and tantalum powders in proportions of 28 atomic % (10% byweight) of Cr and 72 atomic % (90% by weight) of Ta;

into the portion which would have the coldest temperature, a chromiumfluoride CrF₂ powder (5% by weight of the Cr—Ta mixture) carried in agraphite boat.

In a first operation, the ensemble was heated to a temperature of 777°C. (1050 K) in the hot zone and 727° C. (1000 K) in the cold zone, at atotal pressure of 3 kPa maintained by a stream of hydrogen around thesemi-sealed chamber, for 36 hours.

The alloy obtained (or, in a variation, the pure powdered metals in thesame proportions) was used to treat a 3D fibrous carbon preform part of20×5'5 mm³ with 75% porosity suspended above the powder mixture withoutcontact with the latter.

The ensemble was introduced into the middle portion of a jacketed tubefurnace at a temperature of 777° C. (1050 K) such that there was adifference of 50° C. between the hottest temperature where the part tobe treated and the Cr—Ta pack (or in a variation, the powder mixture)were located and the coldest temperature where the solid CrF₂ waslocated. Under these conditions, the theoretical activities of thechromium and tantalum in the pack were a_(Cr)=2.3×10⁻¹ anda_(Ta)=9.9×10⁻¹. The treatment was carried out at a total pressure of2.67 kPa maintained by a stream of hydrogen, for a period of 8 hours. Atthe end of this treatment, an adherent two-phase TaC+Ta₂C coating with ahighly regular morphology had been produced on all of the fibersconstituting the preform. It was of the order of 20 nm thick; itsuniformity was over 90% (said uniformity was the ratio, expressed in %,between the thickness of the coating in the central zone of the preformand the thickness of the coating at the external surface of thepreform).

b) The part coated in accordance with Example 3a was introduced into asemi-sealed graphite chamber which had not been used to carry out anycementation treatment; the part was also carried by a graphite tool. Theensemble was maintained at a temperature of 1300° C. (1573 K) for 48hours at atmospheric pressure maintained by a stream of helium. At theend of this treatment, a mono-phase TaC coating was obtained whichretained the thickness, uniformity, adherence and regular morphology ofthe two-phase TAC+Ta₂C coatings obtained in Example 3a.

EXAMPLE 4

Thermochemical treatment outside the pack, using a MgF₂ activatingcompound and generating layers of boron carbide on three-dimensional(3D) fibrous carbon preforms with 60% porosity.

The following were simultaneously introduced into a semi-sealed graphitechamber:

into the portion which would have the hottest temperature, a mixture ofboron and magnesium powders in proportions of 95.3 atomic % (90% byweight) of B and 4.7 atomic % (10% by weight) of Mg, and a 3D fibrouscarbon preform part of 15×10×5 mm³ with about 60% porosity, suspendedabove the mixture of powders by a graphite tool so that there was nocontact between the two items:

in the portion which would have the coldest temperature, magnesiumfluoride MgF₂ crystals (20% by weight of B—Mg mixture) carried in agraphite boat.

The ensemble was introduced into the middle portion of a jacketed tubefurnace at a temperature of └227° C. (1500 K) such that there was adifference of 70° C. between the hottest temperature where the part tobe treated was located and the coldest temperature where the solid MgF₂was located. The treatment was carried out at a total pressure of 1.33kPa maintained by a stream of hydrogen, for a period of 18 hours. A Mg—Balloy was formed as the temperature in the furnace rose, by melting ofthe magnesium from 650° C. (923 K). Under these conditions, thetheoretical activity of the boron was maintained at 1 and the activityof the magnesium was less than about 10⁻². At the end of this treatment,an adherent rhombohedral boron carbide coating with a regular morphologyhad been produced on all of the fibers constituting the preform; itsthickness varied from 140 nm to 85 nm between the exterior and thecenter of the part.

EXAMPLE 5

Thermochemical treatment carried out in the pack, generating layers ofboron carbide on carbon/carbon composites with 10-15% porosity.

The composition of the B—Mg powder mixture and its disposition withrespect to the solid activating compound MgF₂ were the same as inExample 4. The part to be treated was a carbon/carbon composite of25×8×20 mm³ with 10% to 15% porosity; it was placed inside the powdermixture. The thermochemical treatment was carried out under the sameconditions as those of Example 4, the duration being increased to 32hours and with a total pressure of 0.67 kPa. At the end of thistreatment, the part was extracted from the pack and was easily freedfrom the residues thereof using a brush with flexible bristles of asynthetic material. For this “cleaning”, there was no need to carry outan expensive operation such as machining since the pack did not adhereto the treated part.

An adherent rhombohedral boron carbide coating with a relatively regularmorphology had been produced over all of the porous portions accessibleto the gas phase. Its thickness varied from 1 μm to 0.5 μm between theexterior and the center of the part.

What is claimed is:
 1. Use of an alloy with formula: xE-yM-zM′ where: E is selected from Ti, Zr, Hf, Ta, Nb, Cr, Si, B and alloys thereof; M is selected from Al, Ca, Cr, Y, Mg; M′ is selected from Fe, Ni, Cr, Co, Mo, W; x, y and z represent the atomic percentages of each of said elements E, M, M′;  where x≠0 y≠0 and z=0 or z≠0, provided that: if E=M=Cr, then z≠0 and M′≠Cr; if E=Ti, Zr, Hf, Ta or Nb and M=Cr, then z≠0 and M′≠Cr, as a pack for the thermochemical treatment, at a reduced pressure, in the range 0.1 kPa to 30 kPa of hydrogen, a rare gas or a mixture of these gases, in a halogenated atmosphere, of a carbon-containing material optionally having an open porosity.
 2. Use of the alloy of claim 1 as a pack for the thermochemical treatment, in a halogenated atmosphere, of a carbon-containing material optionally having an open porosity, said treatment comprising maintaining said material: a) at a temperature in the range 700° C. to 1300° C.; b) at a reduced pressure, in the range of 0.1 kPa to 30 kPa of hydrogen, a rare gas or a mixture of these gases; said treatment also being in the presence of a solid activating compound having vapor pressure at said treatment temperature lower than said reduced pressure, with formula MX_(n), where X consists of chlorine or fluorine.
 3. Use of the alloy of claim 2, wherein said carbon-containing material has an open porosity in the range of 2% to 98%.
 4. Use of the alloy of claim 3, wherein said carbon-containing material consists of a graphite part, a completely or partially densified carbon/carbon composite, a non densified fibrous carbon preform, a non densified preform based on SiC fibers pre-treated or non pre-treated to produce a superficial carbon layer, or a carbon foam.
 5. Use of the alloy of claim 2, wherein said carbon-containing material is at least partially immersed in said pack or maintained in the gas phase, in a zone near to said pack.
 6. The use of the alloy of claim 2, wherein said solid activating compound is in contact neither with said pack nor with the treated carbon-containing material.
 7. The use of the alloy of claim 6, wherein said solid activating compound is maintained at a temperature which is 20° C. to 200° C. lower than the temperature of said treated carbon-containing material.
 8. The use of the alloy of claim 2, wherein said thermochemical treatment is carried out: at a temperature of less than 1050° C.; at a reduced pressure of helium or argon; in the presence of a donor Zr—Al pack and an AlF₃ activating compound, to generate a zirconium carbide coating.
 9. The use of the alloy of claim 2, wherein said thermochemical treatment is carried out: at a temperature of less than 1050° C.; at a reduced pressure of hydrogen; in the presence of a donor Ta—Cr and an CrF₃ activating compound, to generate a two-phase TaC+Ta₂C coating.
 10. The use of the alloy of claim 9, wherein said thermochemical treatment comprises an additional annealing heat treatment, carried out at a temperature close to 1300° C., to generate a mono-phase TaC coating.
 11. The use of the alloy of claim 2, wherein said treatment is carried out: at a temperature of less than 1100° C.; at a reduced pressure of hydrogen; in the presence of a donor B—Mg or B—Y pack and an MgF₂ or YF₃ activating compound, to generate a boron carbide coating. 